I have 6 players and an observer. They understand they need to have their turns sorted in combat and defer their turn when they need time to think.

All take part in roleplay, while not equal I do not have any "quiet players".

The problem with a team of 6, so far only 5 have been present at a time, they are breezing through encounters that should be hard.

opening encounter was 5 bandits 25xp × 5people × 2group modifier = 250 xp encounter, a hard encounter. They killed them all before taking a single hit, while the room was also on fire making them take dex saves.

A Knight level 3 encounter was killed easily. Even a centaur went down in two rounds. I thought that it would be more challenging.

The party is all level one yet it is handily slaughtering hard encounters. They work in pairs so that two are stood next to each other on each creature giving them advantage on attack rolls (as I understand it this counts as flanking, two PC both within 5 feet of the same creature), and both clerics have AC 18 making the party tough as nails.

The numbers in the book are not giving correct encounter difficulty, so how should I adjust them for a party of 6?

6 Answers
6

The XP thresholds for 1st level are 25/50/75/100 for Easy/Medium/Hard/Deadly. Given that you've calculated it at 250XP for 5 PCs, that puts it at 50XP/person. That's barely a medium encounter. Remove just one of those guys and it's now an easy encounter. Try adding in another 4 guys and see how it goes.

The centaur, while CR 2, is only 450 XP, which is still only a hard fight. For single monsters, it's easy to focus them down, especially when you're getting free advantage for no reason and they're not.

It really just depends on who you have in the group. If the suggested encounters aren't working for you, try making it more difficult. PC's are much tougher than you think, and can succeed at what you think will kill them all. For instance, when my group was about 7th level, our DM set a Hill Giant after us. We dispatched it after much hardship and thought that was it. But, while the casters where regaining spell slots and the fighters where healing, a second hill giant appeared before we got the rest. You need to experiment, and test the groups limits. If they can take something down immediately after it's first attack, make it's first attack stun them or make their strikes do less. After a single moderately challenging pack of mobs, make them fight another. You eventually learn what the party is weak to, and make them face it.

First, I'd like to clarify some points before actually answering your question. The following first section is a rip-off from my answer here.

The difficulty of encounters (based on EXP) from DMG is highly overestimated

From my experience, either I'm awful playing NPCs at combats or the guidelines on encounters are intended for a really safe world. I've run lots of "Deadly" encounters where at worst my party would deplet their resources, but hardly (even ONE PC) would die from it.

This is specially true for the Adjusted XP, mainly for hordes of CR-lower-than-1 monsters

Example with Spoilers from The Death House (one-shot adventure that is a hook to Curse of Strahd, also regarded one of the hardest published adventures as far as I'm aware) ahead:

Near the end, the players are put against a CR5 Shambling Mound as a bunch of level 2 players. For a 4-PC party of lvl 2 players, a deadly encounter is 800XP - a CR5 alone is 1800, which is almost the entire adventuring day (2400XP). By the time they get there, they usually have used some of their resources due to numerous previous combats. As far as I remember the adventure, they were supposed to just run from it. The Barbarian decided he did not want to and they fought it. After many rounds, the Barbarian dropped to 0 HP and the Mound went for the other PCs. Then they started to kite it (mound moves slower, even with Dash it still closes up slowly) and eventually defeated it. TL;DR: Four 2nd level adventurers can defeat a CR5 monster.

Your Math is off.

As mentioned in Kyle W's answer, 250XP is actually barely medium and 450 XP is only hard. If you want an actually challenging encounter, you should aim at least Deadly.

How to make fights more challenging?

Obvious answer: increase the number of monsters and their CRs. As I mentioned, even a Deadly encounter is rarely actually deadly. At full resources, a low-level party can handle a Twice-their-deadly-threshold encounter, from my experience.

Give the party's enemies tactical advantages - surprise, getting the party on a bad formation, hitting their squishies first, partial or total cover while the party is on open field. As the DM, you create the encounters and you can use everything in your favor.

Multiple encounters without resting - deplet their resources. The DMG also has guidelines on this, as soon as I have the book in hands I'll paste it, but the thing is: Multiple encounters are harder than the individual parts, if they are done without allowing even short rests between them.

For multiple encounters, if you really want to kill them (or make them think they are dying), first take out their resources with Medium/Hard fights, then throw the Deadly fight at them. This ordering, instead of the opposite (first deadly then medium/hard) increases the chance of things going bad if your party plays it wrong (i.e. uses too many resources in the easier fights).

Don't pull your punches. The CR of the creatures in MM is based on using the most-damaging ability available every turn. If you decide to use a weaker one (or if you fudge dice rolls) it becomes easier. Period.

When you have six PCs

The DMG states

If the party contains six or more characters, use the next lowest multiplier on the table. Use a multiplier of 0.5 for a single monster.

This change should be enough to handle the balancing when adding your next PC.

Side note: Flanking is an optional ruling...

... that makes larger groups even stronger. If you want to use this rule with a large number of PCs, you will have to take this into account. I'm just noting it so you might want to open a follow-up question specifically on this. I don't use flanking rules, so I don't know how to balance them, sorry.

\$\begingroup\$OP's interpretation of flanking is also wrong. They're interpreting it as akin to a Wolf's Pack Tactics trait, but the DMG's optional rule for flanking requires that the characters be on opposite sides of the enemy to grant advantage. (That rule still makes it trivial to gain advantage, though, since characters can move around an enemy but stay within its reach to avoid taking opportunity attacks as they reposition - which is why I never use it.)\$\endgroup\$
– V2Blast♦May 4 '18 at 18:54

Use your observer as assistant game master.

Groups of 4+ players who really want to be involved in role-playing are a challenge. Divide and conquer is an option, if you can involve your observer as 'co-game master', so that you can split the group (fighting on the hallway as well as inside a room etc.) and thus only require a smaller amount of monsters and as these constellations of who goes first inside a room, who waited outside, or who went hunting/exploring while the others waited at the camp site might always change and thus make predetermined strategies relying on the whole group present superfluous. They can still exist, but smaller groups with only one crowd-controller or both controllers but only one damage dealer may form. As you can handle the same amount of monsters in less time, with two groups playing at the same time, this gives you additional options.

In particular for roleplaying situations, handing a part of the group to one GM while the rest is handled by the second GM is allowing players to follow their interests and experience interesting times with the other PCs which you could not delve into so much as the rest of the non-involved group would get bored quickly.

The main problem has already been addressed, namely that your party has been getting advantage for no reason, but I wanted to address the specific issue of balancing for six players.

The encounter system for 5e includes XP multipliers based on the numbers of monsters present, but those multipliers weren't written with a six man party in mind. What I'd suggest is treating monsters as if they were in a group one category smaller than they actually are.

1x for two monsters, 1.5x for three to six monsters, etc.

Also, you want to favor groups of monsters over single foes. Generally speaking, a party is going to be ill matched against an enemy who is one or two CR's above them, even if they fall within the XP threshold. While your party might dispatch CR 2s and 3s at first level, a CR 5 Gorgon would likely crush them at fourth level, despite only being between a medium and hard encounter based on their threshold.

One, the dice determine a lot. Those advantage rolls for flanking gave them a huge edge. You should definitely re-read the section about flanking and people have brought up a lot of good points.

Having said that, I've always homebrewed a special rule for flanking. If multiple creatures outnumber players and nearby allies within 5 feet, they get advantage.

This can lead to some chaos as groups pile up, and some move to defend, and it can make an interesting encounter as that last person matters a lot. I find this more fun - but it was definitely creating a huge problem in your game.

Advantage equals a +6 statistically, and it doubles their chance for critical hits from 5% to 10%. Taking away that one rule and then rebalancing your encounters like others have said will be enough.

But you've got a blindspot for the dice. Having played the way you did, beware of overcompensating. Sometimes players play smart, sometimes they roll well - and sometimes they destroy encounters without taking a hit. It's gonna happen.

But when it doesn't, you don't want to ever feel like you went too hard. After you make the adjustments, play it out. The creators have said CR is junk, because it's incomplete. It's not a good measurement. Your cleric heavy team will destroy certain enemies, but something else might hit their weak spot.

Will-o-wisps for instance often do this to parties who rely on martial abilities. Clerics can hit for radiant damage - but if they run into them when low on spell slots, they can be deadly, really deadly. Circumstances sometimes govern the difficulty of encounters and aren't calculable. Is the terrain less than ideal? Is there a puzzle going on? Is a trap going off? Is a friendly npc in danger?

Try tweaking those dials before encounters without changing the CR. You'd be surprised how fighting on a slope can change everything, even if it's walkable.

And then be patient. Inevitability will happen - they will pick the wrong place or the wrong strategy and they will pay for it dearly, and your patience will pay off when they beg, bargain and plea, and you know you just let time and tide govern the moment.